Radiometry system

ABSTRACT

This disclosure relates to a system for detecting metal in a background of terrain or water. It employs a radiometric receiver with input port means directed toward the area to be examined and responsive to thermal emission in two planes of polarization. The system includes means for displaying or comparing the relative emission temperatures in the two planes as an indication of the presence or absence of metal. Fundamentally, the system supresses false alarms due to water.

u ited States Patent Inventor Austin Mar-don [56] References Cited Sam!Barbara, UNITED STATES PATENTS QY J' 9' 3322 3,028,596 4/l962 McGillemet al. 343/100 gf 197] 3,230,532 1/1966 Whitney. H 343/100 AssigneeAerojebcenefal corporation 3,235,731 2/1966 Sellng.. r 343/100 UX ElMonte, Calif. Primary Examiner-Rodney D Bennett, Jr.

Assistant ExaminerTr H. Tubbesing Atlameys-Edward O. Ansell, D. GordonAngus and Arthur Dicker RAD'OMETRY SYSTEM ABSTRACT: This disclosurerelates to a system for detecting 2 Chi 7 Drawin g metal In a backgroundof terrain or water. It employs a 8 radiometric receiver with input portmeans directed toward US. Cl 34 /100, the area to be examined andresponsive to thermal emission in 250/833 two planes of polarization.The system includes means for dis- Int Cl G0 lj 5/10, playing orcomparing the relative emission temperatures in the GOlk 11/00 twoplanes as an indication of the presence or absence of Field olSearch343/100; metal. Fundamentally, the system supresses false alarms due250/83.3 to water.

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AUST u MAROON RADIOMETRY SYSTEM BACKGROUND OF THE rnvrzrmou Themicrowave radiometer has been recognized for many years as effective inmeasuring the emissivities of objects or materials in the field of theradiometer antenna. The microwave radiometer is in actualityanultrasensitive electromagnetic .receiver which responds to thermalenergy generated by all materials above absolute zero. Through the yearsnumerous researchers have cataloged the emissivities of differentmaterials under different conditions and this data has been published..In' general, the emissivity coefficient at microwave frequencies variesfrom. approximately 0.05 to 0.3 for water and metal, while normalterrain features such as earth, gravel, grass, wood or asphalt haveemissivities that range between 0.65 and unity with most of the lattermaterials in the 0.95 to 0.98 range. As a result, water and metal whenviewed by a radiometer, appear as cold target'while other terrainfeatures appear relatively hot."

Most radiometers in use are of the type'commonly referred to as the,Dicke radiometeror signal modulated radiometers.

.They are based upon the teaching of Professor Dicke in his article, TheMeasurement of Thermal Radiation at Microwave Frequencies, The Review ofScientific Inst, Vol. 17, No. 7, July 196, Pg. 268. Basically, the Dickeor signal modulated radiometric receiver is one including a radiofrequency switch between the antenna or input port and thereceiver. Theswitch alternately applies the antenna output and a reference load ofknown emissivity or temperature to the input of the receiver. Thereferenceinputis used to cancel any receiver gain instability. Inaccordance with the teaching of Professor Dicke, it is possible tocontinuously calibratetheradiometer receiver performance against theknown standard and thereby to be able to obtain certain absolute datarelative to the field I of view of the antenna.

SUMMARY OF THE INVENTION I have determined that often the absoluteemissivity or temperature of thefield of view is of less significance tothe observer than anomalies which may appear in the field. Anoma- ;liesof the most obvious type are those features which appear -to be cold ina hot background or vice versa. For example, in navigationalapplications, common visual landmarks are bodies of water. From the data-:tored above, while soil, vegetation and many manmade structures appearas 'hot targets] have,

previously gathered and moniit is clear that water constitutes a coldtarget therefore, determined that a radiometer designed to look at thesurfaces of the earth as from an aircraft can produce usablenavigationalinformation in areas containing significant amounts of water features.Moreover, I have determined that it is possible, not only todiscriminate between hot and cold targets, but that it is possible todiscriminate between two typesof cold targets which are of particularsignificance in the navigational and/or military operations.Specifically, it is possible to discriminate between bodies of waterandmetal in which both appear as cold targets. This is accomplished inaccordance with this invention by means of a radiometer which is capableof receiving energy in two discrete polarizations identified for clarityas horizontal and vertical. It has been determined that the emissivityof metal, a cold target, is virtually the same in both horizontal andvertical polarization planes. However, water targets, by way ofcontrast, have a difierent apparent temperature in the horizontal andvertical polarities. In particular, in the horizontal polarizationplane, water targets appear cold like metal. However, water appearssomewhat warmer in the vertical polarization plane. This discriminationis accomplished by use of a radiometer simultaneously or sequentiallyviewing the field in-the horizontal and verticalipolarization planes.Suitable switching circuitry is provided whereby the apparenttemperaturesofthe'same field in both thezvertical and horizontal planesis represented or identified. Where the vertical and horizontalpolarized energy of a cold target is at the same apparent temperature,the target can be identified as metal.

Based upon the above recognition, I have designed radiometer systemscapable of distinguishing metal from nonmetallic targets in the presenceof other natural or manmade materials.

In one embodiment of this invention,.a single polarized port or antennais mounted to view the required field and rotated about the viewing axisto present alternately horizontal and vertically polarized views of thefield. Synchronized with the rotating port or antenna is switchingcircuitry to discriminate between information obtained in eachpolarization. A recorder or display continuously records or shows theapparent temperature detected. An observercan readily distinguish when acold target exhibits no significant difference in the horizontal andvertical plane indicative of the presence of a metal object in the fieldof view. For convenience, this embodiment of the invention is identifiedas a single channel time multiplexed dual polarization system.

In another embodiment of this invention (which requires additionalelectronic circuitry and stabilization, but eliminates the rotatingapparatus) a pair of polarized antennae or ports is Description of theDrawing This invention may be more clearly understood from the followingdetailed description and by reference to the drawing in which:

FIG. 1a is a tabular representation of the emissivity of variousmaterials;

FIG. lb is a tabular representation of the apparent temperatures ofsimilar materials as subject to examination in different planes ofpolarization;

FIG. 10 is a graphical representation of the temperature of metal andwater targets in difierent polarizations as a function of apparenttarget size;

FIG. 2 is a block diagram of a single channel time multiplexedradiometer in accordance with this invention; and.

FIG. 3 is a block diagram of a dual channel radiometer designed todistinguish between metal and nonmetallic objects; and FIGS. 4 and 5 areforms of signal processor 64 of FIG. 3.

DETAILED DESCRIPTION OF the INVENTION Now, referring to the drawing FIG.la the emissivity of different materials at microwave frequency isrepresented. It may be seen that salt water, fresh water, and metallicobjects tend to have lower emissivity in the range of 0.05 to 0.3 and,therefore, are categorized as cold targets. This is true throughout thefrequency range of approximately 10 to GI-lz. Similarly, organicmaterials plus natural terrestrial structures, tend to have emissivitiessignificantly higher in the range of 0.65 to unity and, therefore, areconsidered as hot" targets.

The difference in emissivities their these materials affect theirapparent temperatures when viewed by a microwave radiometer. For thecase of a theoretical black body, it will emit radiation as a smoothfunction of wavelength and its apparent temperature will be the sameasmeasured by a thermometer. Real objects of different materials andsurface conditions are not perfect black bodies so their emission is afunction of both thermometric temperatures and emissivity. Theemissivity by definition equals 1 minus the materials reflectively sothat we find highly reflective surfaces such as exposed metal and stillwater appearing very cold, efficiently reflecting the'cold sky.

Normal terrain materials, b rock, sand and vegetation have emissivitieswhich vary widely depending on moisture content, compactness and thematerial as may be seen in FIG. la. However, the emissivity of all ofthese materials is significantly higher than water and metal. The effectis apparent from FIG. lb where the apparent temperatures of a number ofmaterials as measured at 17 GI-Iz. is tabulated. These measurements weremade on materials out-of-doors exposed to a cold sky and viewed from anincident angle of 22 from the vertical.

Referring to the first numerical column of FIG. 1b, it is apparent thatmetal and water have apparent temperatures of l200 cooler thanvegetation, soil and asphalt. Therefore a radiometric system which willdetect this difference in apparent temperature can distinguish betweenwater or metal as cold targets and vegetation, soil, or other materialsas hot targets.

The first numerical column was measured with a radiometer in which theport responds to energy in only one plane of polarization. The secondnumerical column records the apparent temperature in a plane nonnal tothe first plane. For purposes of convenience these are labeled thehorizontal H and vertical V planes using the orientation of the antennaports as the plane of reference. Note that in all cases except metal theapparent temperature of the materials is lower in the horizontal than inthe vertical plane. The difference in the case of water is in the orderof 50 when the target fills the antenna aperture This characteristic isalso illustrated graphically in FIG. with both metal and water targetsviewed against a background of vegetation. The ordinate axis of FIG. 10records apparent temperature in degrees Kelvin and the abscissa, theapparent target size to the viewing port. This apparent target size, ofcourse, is a function of target actual size, antenna port size andtarget range.

From FIG. 1c it is apparent that metal targets appear somewhat colderthan water in the vegetation background at all significant target sized(0.3 and up) but more important is the insensitivity of metal topolarization plane of viewing. This is denoted by the nearly coincidenthorizontal and vertical curves for metal and divergent horizontal andvertical lines for water.

Based upon the foregoing I have devised a system for distinguishingmetal targets in backgrounds of water, vegetation or other materials.

In FIG. 2, a block diagram of a system in accordance with this inventionmay be seen. It includes a single Casscgrainian antenna 10 with a majorreflector 11, for example 36 or 48 inches in diameter with a suitableradome 12 shown in dashed lines, a subreflector l3 and a receiving port14 at the focus of the antenna 10. The port 14 is rectangular andsensitive to energy in one plane of polarization.

The antenna 10 or at least the port 14 is mounted for rotation about theaxis of the antenna. The port 14 is electrically coupled through aturnstile junction 15 to sample the horizontal and vertical componentsthrough leads or 21 to respective circulator switches 22 and 23 whichserve as radio frequency modulators. The switches are conventionalferrite type circulators having a forward insertion loss in the order ofone-fourth of a db. and greater than db. in the reverse direction tomaintain isolation.

The circulator switches alternately sample the incoming vertical orhorizontal component signal as the Hz. may be and a reference load 24which may be a matched coaxial load held at Y. constant temperature forexample 333K2DJ K. The temperature of the reference load is preferablymonitored continuously by a precision platinum thermometer which outputis used to regulate the heater temperature and thereby provide aconstant temperature environment for the reference load 24.

The circulator switches 22 and 23 are driven by a common 600 Hz.generator 25 through a power amplifier 26 providing their synchronizedswitching at the 600 Hz. rate. The vertical and horizontal componentsalternating with the reference signal are introduced into respective RFamplifiers 30 and 31 Y which preferably consist of three tunnel diodestages each with the total gain in the order of 48 db. and a measuredsignal-to-noise figure of less than 6db. The outputs of the tunnel diodeamplifiers 30 and 31 are then introduced into respective diode detectors32 and 33 and then into low noise video amplifiers 34 and 35 providinggain in the order of 60 db. sufficient to drive synchronous demodulatorsincluding phase detectors 36 and 37 driven in synchronism with thecirculator switches by the 600 Hz. reference generator. These videoamplifiers preferably consist of two cascaded stages with sufiicientfeedback to stabilize the amplifier AC gain and DC characteristics dueto aging or environment. Amplifiers having a bandwidth in the order of30 to 600 Hz. less than 4db. will provide suitable video amplificationfunction.

The composite signals are synchronously demodulated by phase detectors36 and 37 and the varying DC output of each channel is amplified inoperational amplifiers 40 and 41 and applied as the input signal to adual channel chart recorder 42. Two pens 43 and 44 print traces of theapparent temperature of the field of view in the vertical and horizontalpolarization planes side by side or if preferred overlying each other.Temperature is measured transverse to the direction of movement of thechart 45.

There may be seen on the chart 45, the simplified represen: tation of ashort continuous observation run with two pens transcribing generallyparallel paths with two anomaly recorded. The first anomaly appearsalong the reference line a" where (1) both traces are in the lower orcold" region and (2) the vertical polarization component appears at thesame temperature as the horizontal polarization component.

In accordance with the criteria set forth above, this is indicav tive ofa metal target. The water background is typified by the region b." Theregion designated by reference c has a higher apparent temperatureindicative of land mass or vegetation. At position reference d a coldobject is present in a hot background. Again the vertical polarizationcomponent is of equal temperature with its horizontal counterpart againindicative of the presence of metal against a natural background, inthis case terrain or vegetation, in all probability,. The system asdescribed above employs a skilled operator who continuously monitors thetraces and visually distinguishes between hot and cold targets whilecomparing the relative level in the H and V planes of cold targets todetect the presence of water.

This system is fully effective. However, the observation and analysissteps can also be performed automatically employing electrical circuitryof well-known design and with the elimination of operator fatigue.Additionally the elimination of mechanical parts can be advantageousBoth of these advantages are described in the system of FIG. 3.

Now referring to FIG. 3 a pair of antenna ports 50 and 51 may be seen,polarized and oriented normal to each other and carried on a vehicle,(unshown in the drawing) to scan a common field. A typical carryingvehicle for the system is a helicopter for aerial reconnaissance orsurveillance.

The two polarized antennae are connected via wave guides 52 and 53 orequivalent to respective radio frequency switches 54 and 55 each drivenby a common reference signal generator 56 in opposite phase. Thehorizontally polarized signal from antenna 50 is applied through switch54 to an RF amplifier 60 and detector 62 to a signal processor 64.Similarly the vertical component sensed by antenna 51 is applied throughswitch 55 and an RF amplifier 61 and detector 63 to the same signalprocessor 64. The reference generator may include a temperaturereference R or may simply provide a switching operation depending uponthe type of signal processing to be used.

The input signals to the processor 64 are preferably unidirectionalvoltages varying as a function of the apparent temperature of the fieldas viewed. The signal processor 64 may combine the input signals in avariety of ways using wellknown electronic techniques to performmathematical transformation to derive the significant information fromthe two channels. The simplest transformation is the subtraction of theH from the V component. This is the same operation which the and a noisefig. of i operator using the apparatus of FIG. 2 performs mentally. Itcan be accomplished by a simple difierential 64a shown in FIG. 4 whichmay be substituted for the box 64 of FIG. 3.

To minimize the effects of change in background levels a moresophisticated form of signal processing may be used. This is illustratedby the network 64b of FIG. 5. In this case the average of the horizontaland vertical polarization components V+Hl2 is derived and divided by thedifference V-H. Using this form of processing not only is the effect ofaverage level variations minimized but the significant variable, namely,the V-H factor is in the denominator. As' V-H approaches zero, thequotient approaches infinity. This relationship the output signal is +HF=R 2 \s V H The advantage in the use of these more s6phisticated formsof data processing is apparent in FIG. 1b in the right-hand column ascompared with the column V-H. Given the measurements V and H as theyappear in the two columns to the left of the double lines the extractionof the difference value V-I-I shows a detectable difference between thecharacteristics of metal as compared to water and other terrainfeatures. However, all V-H factors are in one or two orders of magnituderanging from 0 for metal to 56 for water to 7 for vegetation or soils.Processed in the manner shown in the two right-hand columns of FIG. 1bthe distinction betweenmetal and all other materials is of suchmagnitude that a fixed threshold of alann can be built into the dataprocessor to signal metal detection with minimum danger of false alarms.

The signal processor is connected to a utilization device which may be arecorder of the type shown in FIG. 2 or simply an alarm. In some casesit may be advisable to feed the output of this system into anavigational or other type of system for control of the vehicle.

I claim:

1. In a radiometer system in which signals are derived representingemissivities of -a surface with regard to different sensingpolarizations, means to minimize the effect of variations in targetbackground emissivity on the signals, comprismg:

first means to generate a signal representing the average of the derivedsignals;

second means to generate a signal representing the difference betweenthe derived signals; and

means to generate a signal representing the quotient of the signalsgenerated by said first and second means.

2. The system of claim 1 and means to generate a signal. representingthe emissivity of a reference target; and

means to combine the reference signal with the derived signals toprovide input for said first and second means.

1. In a radiometer system in which signals are derived representingemissivities of a surface with regard to different sensingpolarizations, means to minimize the effect of variations in targetbackground emissivity on the signals, comprising: first means togenerate a signal representing the average of the derived signals;second means to generate a signal representing the difference betweenthe derived signals; and means to generate a signal representing thequotient of the signals generated by said first and second means.
 2. Thesystem of claim 1 and means to generate a signal representing theemissivity of a reference target; and means to combine the referencesignal with the derived signals to provide input for said first andsecond means.